Saturn’s moons create gaps in the planet’s rings; by studying them, astronomers can discern the presence of the moon. But when this happens around stars, a new force enters the picture.
Over two decades ago, scientists confirmed the first-ever detection of a planet outside the Solar System – an exoplanet. With it, the possibility of finding more potentially habitable worlds catapulted the planet-hunting endeavour into the public imagination. Meanwhile, astronomers kept discovering one exoplanet after another, all of them within the Milky Way.
It was splendid to realise that our host galaxy itself was home to so many worlds of interest, but whether other galaxies also housed such worlds has remained a matter of theory. Recently, however, this speculation was called up when astronomers reported that they had perhaps found an exoplanet in another galaxy.
Hunting for planets is not easy. Even with our most advanced telescopes, the chances of capturing an image of an exoplanet are slim at best. Most planets are minuscule compared to the much-bigger and bright stars they orbit. The starlight reflected by planets thus becomes lost in the blinding light of the star itself. This makes it difficult to detect exoplanets by ‘direct’ imaging. So astronomers came up with a few tricks. One of them infers the presence of planets by looking at patterns formed by dust and debris around a star.
“Gravitational perturbations from planets open gaps and excite spiral density waves in disks, and so these features are inferred to indicate the presence of planets,” James Stone, an astrophysicist at Princeton University, told The Wire. This is similar to what happens in Saturn’s rings – where spaces are created in the dusty disks and patterns enforced by the planet’s moons.
Elegant as this method is, it seems they might not always be reliable because, it turns out, these patterns could form without planets, too. A recent study to be published in the Astrophysical Journal found that similar patterns can be created by a combination of forces, not including an unseen planet’s gravitational pull.
Dust and gas coexist as they whirl around a star. When an energetic beam of radiation hits an atom inside a dust particle, it can knock off an electron. The liberated electron can in turn can transfer its energy to a nearby gas particle, heating it. As this happens billions of times over, the gas heats up and creates a high pressure region which acts as a sink for dust particles. So more dust particles shed more electrons and this leads to an increasing heating of the gaseous region.
This cascading effect – the photoelectric instability – is a major player in creating a variety of patterns in the dust-gas area: clumps of dust, disks, rings, arcs and spirals. These structures could potentially reveal information about an unseen planet.
“The width of the gap, and the amplitude and pitch angle of spiral waves, can tell us about the mass of the planet that produces them,” Stone, who was not involved in the study, said. “Similarly, the phase of the spiral wave can indicate at what azimuthal angle the planet is located.
By studying the geometry of these features, astronomers pick up clues about where in the disk a planet could be located, its approximate mass and, as a result, how bright it could be.
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Then again, assuming these structures are being influenced by an embedded planet could prove to be false. “I feel we are sometimes too quick to jump to the idea that the structures we see are caused by planets,” Wladmir Lyra, one of the researchers involved in the study, said in a statement. “That is what I consider an extraordinary claim. We need to rule out everything else before we claim that.”
The model proposed by Lyra and his peers has many details often used in astrophysics studies at this scale. What’s new is that they’ve also included the radiation pressure force: the force generated by a star-born photon impinging on a small dust particle. This force plays a negligible role in the daily lives of humans but in space, it can drastically alter the dynamics of debris by knocking tiny dust grains out of their orbits. How photoelectric instability and radiation pressure work together depends on the properties of dust grains in the debris, especially their size and distribution.
To understand how a system comprising a star surrounded by a disc of gas and dust evolves in time, the Lyra and co. conducted high-fidelity simulations on a supercomputer. The underlying code predicted the motion of gas as dictated by the Navier-Stokes equations for fluids and also kept track of the motion of dust particles. By changing the amount of dust and the size of dust grains, the authors could control the strength of the radiation pressure relative to the photoelectric instability.
Their simulations were able to replicate a number of patterns that have been observed in different star systems. More importantly, all patterns were formed without having to factor in the attractive pull of an exoplanet.
Because there’s a tug-of-war between the opposing effects of photoelectric instability and radiation pressure, fiddling with the system’s parameters lead to the formation of structures that previous studies couldn’t explain. It seemed that while an exoplanet could have induced the patterns in some instances, the new planet-free model provided a good alternative in many others and, in a few, even a better one.
Alexander Richert, an astrophysicist involved in the study, said in the same statement, “People very often model these systems with planets, but if you want to know what a disk with a planet looks like, you first have to know what a disk looks like without a planet.”
In other words, this study has decoded the forces responsible for sculpting the dust around a star into recognisable patterns. Since exoplanets are still a hot topic in astronomy, studies like this can lead to the formation of better theories and call for more experiments. The research is also a good example of why questioning a prevalent and existing scientific method can’t only be beneficial but enlightening as well.
Ronak Gupta recently completed his masters in fluid mechanics from the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore. He writes about all things science.